Re: [RFC PATCH v3 0/5] sched: support schedstats for RT sched class

[Yafang Shao]

On Sat, Nov 28, 2020 at 12:12 AM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>
> We want to measure the latency of RT tasks in our production
> environment with schedstats facility, but currently schedstats is only
> supported for fair sched class. This patch enable it for RT sched class
> as well.
>
> - Structure
>
> Before we make schedstats available for RT sched class, we should make
> struct sched_statistics independent of fair sched class first.
>
>
> The struct sched_statistics is the schedular statistics of a task_struct
> or a task_group. So we can move it into struct task_struct and
> struct task_group to achieve the goal.
>
> Below is the detailed explaination of the change in the structs.
>
> The struct before this patch:
>
> struct task_struct {            |-> struct sched_entity {
>     ...                         |       ...
>     struct sched_entity *se; ---|       struct sched_statistics statistics;
>     struct sched_rt_entity *rt;         ...
>     ...                                 ...
> };                                  };
>
> struct task_group {             |--> se[0]->statistics : schedstats of CPU0
>     ...                         |
>  #ifdef CONFIG_FAIR_GROUP_SCHED |
>     struct sched_entity **se; --|--> se[1]->statistics : schedstats of CPU1
>                                 |
>  #endif                         |
>                                 |--> se[N]->statistics : schedstats of CPUn
>
>  #ifdef CONFIG_FAIR_GROUP_SCHED
>     struct sched_rt_entity  **rt_se; (N/A)
>  #endif
>     ...
> };
>
> The '**se' in task_group is allocated in the fair sched class, which is
> hard to be reused by other sched class.
>
> The struct after this patch:
> struct task_struct {
>     ...
>     struct sched_statistics statistics;
>     ...
>     struct sched_entity *se;
>     struct sched_rt_entity *rt;
>     ...
> };
>
> struct task_group {                    |---> stats[0] : of CPU0
>     ...                                |
>     struct sched_statistics **stats; --|---> stats[1] : of CPU1
>     ...                                |
>                                        |---> stats[n] : of CPUn
>  #ifdef CONFIG_FAIR_GROUP_SCHED
>     struct sched_entity **se;
>  #endif
>  #ifdef CONFIG_RT_GROUP_SCHED
>     struct sched_rt_entity  **rt_se;
>  #endif
>     ...
> };
>
> After the patch it is clearly that both of se or rt_se can easily get the
> sched_statistics by a task_struct or a task_group.
>
> - Function Interface
>
> The original prototype of the schedstats helpers are
>
> update_stats_wait_*(struct cfs_rq *cfs_rq, struct sched_entity *se)
>
> The cfs_rq in these helpers is used to get the rq_clock, and the se is
> used to get the struct sched_statistics and the struct task_struct. In
> order to make these helpers available by all sched classes, we can pass
> the rq, sched_statistics and task_struct directly.
>
> Then the new helpers are
>
> update_stats_wait_*(struct rq *rq, struct task_struct *p,
>                     struct sched_statistics *stats)
>
> which are independent of fair sched class.
>
> To avoid vmlinux growing too large or introducing ovehead when
> !schedstat_enabled(), some new helpers after schedstat_enabled() are also
> introduced, Suggested by Mel. These helpers are in sched/stats.c,
>
> __update_stats_wait_*(struct rq *rq, struct task_struct *p,
>                       struct sched_statistics *stats)
>
> - Implementation
>
> After we make the struct sched_statistics and the helpers of it
> independent of fair sched class, we can easily use the schedstats
> facility for RT sched class.
>
> The schedstat usage in RT sched class is similar with fair sched class,
> for example,
>                 fair                            RT
> enqueue         update_stats_enqueue_fair       update_stats_enqueue_rt
> dequeue         update_stats_dequeue_fair       update_stats_dequeue_rt
> put_prev_task   update_stats_wait_start         update_stats_wait_start
> set_next_task   update_stats_wait_end           update_stats_wait_end
>
> - Usage
>
> The user can get the schedstats information in the same way in fair sched
> class. For example,
>                 fair                            RT
> task show       /proc/[pid]/sched               /proc/[pid]/sched
> group show      cpu.stat in cgroup              cpu.stat in cgroup
>
> The sched:sched_stat_{wait, sleep, iowait, blocked} tracepoints can
> be used to trace RT tasks as well. sched_stat_runtime can also be
> supported in the future if it is helpful.
>
> Yafang Shao (5):
>   sched: don't include stats.h in sched.h
>   sched, fair: use __schedstat_set() in set_next_entity()
>   sched: make struct sched_statistics independent of fair sched class
>   sched: make schedstats helpers independent of fair sched class
>   sched, rt: support schedstats for RT sched class
>
>  include/linux/sched.h    |   3 +-
>  kernel/sched/core.c      |  25 +++--
>  kernel/sched/deadline.c  |   5 +-
>  kernel/sched/debug.c     |  82 +++++++--------
>  kernel/sched/fair.c      | 209 +++++++++++++++------------------------
>  kernel/sched/idle.c      |   1 +
>  kernel/sched/rt.c        | 178 ++++++++++++++++++++++++++++++++-
>  kernel/sched/sched.h     |  13 ++-
>  kernel/sched/stats.c     | 105 ++++++++++++++++++++
>  kernel/sched/stats.h     |  80 +++++++++++++++
>  kernel/sched/stop_task.c |   5 +-
>  11 files changed, 517 insertions(+), 189 deletions(-)
>
> --
> 2.18.4
>

Ah, the patch #5 is missed,pls ignore this patchset. I will resend them.

-- 
Thanks
Yafang